1. Field of the invention
The present invention relates to a magnetostrictive transducer for generating and receiving elastic waves in a testing plate and an apparatus for structural diagnosis using the magnetostrictive transducers. More particularly, the present invention relates to a magnetostrictive transducer which may generate and transmit elastic waves using the magnetostrictive effect in any directions without re-attaching ferromagnetic patch on the non-magnetic testing plate and may produce Lamb waves and Shear Horizontal (SH) waves in desired directions, and an apparatus for structural diagnosis using the magnetostrictive transducers.
2. Description of the Related Art
Magnetostrictive effect, which is also called Joule effect, refers to a phenomenon that mechanical deformation occurs in a ferromagnetic material when the ferromagnetic material is placed in magnetic fields. A reciprocal phenomenon of the magnetostrictive effect (i.e. a phenomenon that changes in magnetic field in vicinity of materials occur when mechanical stress is applied to the materials) is designated as inverse magnetostrictive effect or Villari effect.
The magnetostrictive effect can be expressed as follows.
When ferromagnetic materials are placed in magnetic field H, induced magnetic flux density B inside the ferromagnetic materials is the sum of magnetic flux density in vacuum and magnetic flux density induced by magnetization of the ferromagnetic materials. Therefore, the magnetic flux density B inside the ferromagnetic materials can be expressed as equation 1.B=μ0H+μ0M=μ0H+μ0χmH=μ0(1+χm)H=μ0μrH=μH  Equation 1
where B denotes magnetic flux density, H denotes magnetic field intensity applied by an outside magnet or electromagnet, M denotes magnetization, χm denotes magnetic susceptibility, μ0 denotes permeability of free space, μr denotes relative permeability, and μ denotes permeability.
The Villari effect and Joule effect can be expressed as equations 2 and 3.                     ɛ        =                              σ                          E              H                                +                      q            *                          H              .                                                          Equation        ⁢                                   ⁢        2             B=μσH+qσ  Equation 3
where ε denotes strain, σ denotes stress applied to the ferromagnetic material, EH denotes modulus of elasticity in a constant magnetic field and μσ denotes permeability when constant stress is applied.
The coefficients q* and q in equations 2 and 3 representing the Villari effect and Joule effect can be represented as equations 4 and 5, respectively.                                                                         q                *                            =                                                ⅆ                  ɛ                                                  ⅆ                  H                                                      )                    σ                .                            Equation        ⁢                                   ⁢        4                                                                    q              =                                                ⅆ                  B                                                  ⅆ                  σ                                                      )                    H                .                            Equation        ⁢                                   ⁢        5            
Transducers using such magnetostrictive effect are applied in various fields because it is possible for the transducers to measure deformation of objects to be measured without physical contact with the objects. According to the magnetostrictive effect, it is possible to generate elastic waves without physical contact with objects to be measured and to generate elastic waves larger than waves generated using piezoelectric effect in the related art.
Generally, elastic guided waves in a plate can be classified into Lamb waves and SH waves with regard to vibrating way of particles. Lamb waves refer to elastic waves vibrating a particle in a plane, parallel to direction of wave propagation and vertical to a plate, and SH waves means elastic waves vibrating a particle vertically to the direction of wave propagation in a plane parallel to the plate. Especially, since a first mode of the SH waves travel without collision with lower and upper interfaces of the plate, it is non-dispersive and can propagate with high efficiency.
FIGS. 1a and 1b illustrate a general configuration of an apparatus for generating elastic waves in the related art. As illustrated in FIGS. 1a and 1b, the apparatus includes: a thin ferromagnetic strip 1 adhered on a plate 2 using a couplant 4, a plate magnetostrictive probe 5 mounted on the ferromagnetic strip 1.
In the apparatus for generating elastic waves in the related art, the ferromagnetic strip 1 is adhered on the plate 2, and introduction of currents to the probe induces magnetic field with flux lines vertically entering and radially emitting out the ferromagnetic strip 1. Therefore, a change in rectangular ferromagnetic strip through the magnetostrictive effect mainly occurs in a direction of its length. Consequently, once the apparatus for generating elastic waves in the related art is installed, a direction of generated elastic waves is permanently fixed. So, in order to change a direction of elastic waves propagation, it is needed to detach the ferromagnetic strip 1 from the plate 2 and re-attach the ferromagnetic strip 1 on the plate 2 in a desirable direction. However, occasionally, it is impossible to detach and re-attach the ferromagnetic strip when defects of an inaccessible structure are to be inspected. Thus, it is difficult to use the apparatus of the related art for generating elastic waves in a desired direction.
Since maximum deformation occurs in a direction of magnetic field according to the magnetostrictive effect, it is desirable that magnetic field applied to the ferromagnetic strip is generated in a direction of elastic waves propagation. However, the apparatus for generating elastic waves in the related art generates magnetic field with flux lines vertically entering and radially emitting out the ferromagnetic strip, so that it is not effective to enlarge output of the elastic waves.
Moreover, it has a drawback that a plurality of apparatuses for generating elastic waves are required for two dimensional inspection of defects in objects to be inspected, since the direction of elastic waves propagation is fixed once the apparatus in the related art is installed.
Further, the apparatus in the related art has another drawbacks that it needs pre-magnetization every single run and it is difficult to control size of pre-magnetization and elastic waves, because pre-magnetization of the ferromagnetic strip through using permanent magnets should be performed before applying additional magnetic field through the coil in order to generate SH waves.
Therefore, there is a need for a more efficient magnetostrictive transducer which may generate elastic waves in a desired direction without re-attaching a ferromagnetic strip thereof and may generate bias and additional magnetic fields in same direction. Further, there is a need for an apparatus for structural diagnosis using such transducers which may perform two dimensional inspection of defects in a wide plate.